Fifteen years ago in California, a tall technology geek named Steve Mushero started writing a book that predicted the American dream might soon “be found only in China.” Before long, Mr. Mushero moved himself to Shanghai and launched a firm that Amazon.com Inc. and Alibaba Group Holding Ltd. certified as a partner to serve the world’s biggest internet market. These days, the tech pioneer has hit a wall. He’s heading back to Silicon Valley where he sees deeper demand for his know-how in cloud computing. “The future’s not here,” said the 52-year-old.